Tubular elements having a closed end and composed of a solid electrolyte for use in the determination of the oxygen content in gases, particularly exhaust gases from internal combustion engines, are known (see, for example, said US Pat. No. 4,021,326). These known elements which consist of zirconium dioxide stabilized with calcium oxide have the inherent disadvantage of only being operative at above 500.degree. C.
It is also known to stabilize zirconium dioxide with yttrium oxide, or ytterbium oxide, or with a mixture of both of these oxides. A so-called yttrium concentrate which in addition to yttrium oxide contains oxides of the heavy rare earth metals, can also be used in place of yttrium oxide as an additive to stabilize zirconium dioxide.
Zirconium dioxide which has been stabilized with yttrium oxide or ytterbium oxide when compared with zirconium dioxide stabilized with calcium oxide, is characterized by an increase of electrical conductivity in the range of one order of magnitude and exhibits a greater degree of ionization. Because of these beneficial characteristics, sensors made from such solid electrolytes are operative at temperatures of approximately 350.degree. C. and higher. Sensors prepared from zirconium dioxide stabilized with ytterbium dioxide have a threshold temperature which is even somewhat lower than said approximately said 350.degree. C.
Technically, it is possible to produce a tubular solid electrolyte element made of zirconium dioxide stabilized with yttrium oxide and/or ytterbium oxide. Because of the high cost of yttrium oxide and the even higher cost of ytterbium oxide when compared with calcium oxide, it is prohibitive economically to produce a zirconium oxide solid electrolyte stabilized with yttrium oxide and/or ytterbium oxide.
Based on a consideration of cost factors, an attempt has been made to join an end plate made of zirconium dioxide stabilized with yttrium oxide to the end of a metal tube. However, because of the difference in physical properties of these two components at the high working temperatures, difficulties arose in connection with the joining and sealing of the yttrium oxide stabilized zirconium dioxide and the metal.